Why are one person's text messages not showing up

I have a ZTE zmax, and am using sms through Facebook messenger. It has worked perfectly for years, but recently for only one of my contacts I get no notification, and I can see the text preview but when I open the conversation his message is not there. The message seems to come through a couple hours to days later. Why aren't I getting a notification and why are his messages taking so long to show up? Thank you!